Blank a space or tab. Word- a sequence of characters considered as a single unit by the shell. Also known as a token. Name

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Week 4

In week 4 we were thoroughly introduced to The Shell.

First we started with some definitions:

blank - A space or tab.

Word- A sequence of characters considered as a single unit by the shell. Also known as a token.

Name- A word consisting only of alphanumeric characters and underscores, and beginning with an alphabetic character or an underscore. Also referred to as an identifier.

Metacharacter -A character that, when unquoted, separates words. One of the following: | & ; ( ) < > space tab

control operator - A token that performs a control function. It is one of the following symbols: || & && ; ;; ( ) |

Reserved words are words that have a special meaning to the shell.

A shell script is a file that contains commands to be executed by the shell. Making a File Executable – use chmod: $ chmod +x scriptname

Read (command) = is used to accept the user input and store the input in variable.

If a word begins with a tilde character (`~'), all of the characters preceding the first slash (or all characters, if there is no slash) are treated as a possible login name.

Command substitution allows the output of a command to replace the command name. There are two forms:

$(command) or `command

Built-in Shell Variables are very useful and important:


Number of command-line arguments.


Options currently in effect (arguments supplied to sh or to set).


Exit value of last executed command.


Process number of current process.


Process number of last background command.


First word; that is, command name.


Individual arguments on command line (positional parameters). The Bourne shell allows only nine parameters to be referenced directly (n = 1-9); the Korn shell allows n to be greater than 9 if specified as ${n}.


All arguments on command line ("$1 $2...").


All arguments on command line, individually quoted ("$1" "$2" ...).

The shift command – promotes each of the command line arguments. $2 becomes $1, the third becomes the second, etc.

The set command - sets the values of the command line arguments

Control Flow Commands – they include control structures:

  • Simple two-way branching If statement

  • Multiple branching Case statement

  • For, While and Until statements

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